In the recovery of hydrocarbons (e.g. oil, natural gas) it is necessary to drill a hole or well into a subsurface or subterranean hydrocarbon bearing formation. The well provides a path for the hydrocarbon to flow from the formation up to the surface. The recovery of hydrocarbons from a subterranean formation is known as “production.” In such productions, a casing is installed in the drilled wellbore to provide a structurally sound conduit for the recovery of hydrocarbon. This is referred to as a cased well. Alternatively, hydrocarbons are retrieved from an uncased or “open hole” well.
It is common practice to use a fluid or mud within the wellbore in order to create a hydrostatic head. This fluid can be weighted to control the hydrostatic head in order to create varying differential pressures between the hydrocarbon formation and the wellbore. The wellbore may be placed in a static underbalanced condition wherein the wellbore pressure is less than the formation pressure. If the wellbore pressure is equal to the formation pressure, the wellbore is said to be in a balanced static condition. An overbalanced static condition is achieved when the wellbore pressure is greater than the formation pressure.
The hydrostatic head is further manipulated to control hydrocarbon production as placing the well in an underbalanced condition will draw hydrocarbon from the formation into the wellbore allowing production of the hydrocarbon to the surface.
In order to complete a well, the wellbore and one or more hydrocarbon formations adjacent the wellbore must be perforated in order to facilitate the flow of hydrocarbon from the formation into the wellbore for production to the surface. Alternatively, in an injector well, perforation of the wellbore and adjacent formations is required for fluid to be injected into the formation from the wellbore. A perforation gun string including a plurality of perforation guns may be lowered into the wellbore such that the perforation guns may be fired to perform the perforating operation. In a cased wellbore, the wellbore casing, in addition to the hydrocarbon formation, requires perforation. Similarly, in an open hole wellbore, a gun string may be used to perforate through the filtercake deposited on the wellbore and into the surrounding formation.
The explosive nature of perforating systems using a string of perforation guns may also damage the adjacent hydrocarbon formation. The firing of the perforation guns can shatter the sand grains of the formation and create rock debris as well as perforation charge debris that may fill or block the perforation tunnels. A shock damaged region of a hydrocarbon formation may have a resulting permeability that is lower than that of a virgin hydrocarbon formation matrix. The extent of the damage to the hydrocarbon formation matrix and the amount of loose debris in the perforation tunnels may be dependent upon a variety of factors including the properties of the hydrocarbon formation itself, the explosive charges themselves, pressure conditions, well fluid, and hydrocarbon properties. The shock damaged region of the hydrocarbon formation and loose debris in the perforation tunnels may impair the productivity of production wells or the injectivity of injector wells.
One known method of obtaining clear perforation tunnels is underbalanced perforating. In underbalanced perforating, the perforating operation is carried out with a static wellbore pressure lower than the formation pressure. After the creation of the perforation tunnels, hydrocarbon initially flows from the formation and through the perforation tunnels thereby clearing some of the debris from the perforation tunnel.
However, underbalanced perforating may be limited in effectiveness, safety, or cost depending upon the hydrocarbon formation and other downhole wellbore conditions. For example, when formation pressure is high and the formation matrix is weak, too large of an underbalanced pressure differential may result in collapse of the perforation tunnels and/or excessive debris production. In another example, an underbalanced well after perforation presents control and safety issues regarding the extraction of the spent perforation gun from the well while the well is in the underbalanced condition. This results in the need for additional specialized equipment and longer work times in order to complete the perforation and gun extraction.
Perforating may also be performed in an overbalanced static wellbore state that improves upon some of the negative aspects of underbalanced perforating, as noted above. However, the benefits of overbalanced perforating are typically overshadowed by a significant loss in productivity due to increased damage to the hydrocarbon formation and perforation tunnels. This negative impact on productivity has typically made underbalanced perforating a preferred choice for well completion.
The recent introduction of dynamic underbalanced perforating limits the negatives associated with both overbalanced and underbalanced perforating techniques. In dynamic underbalanced perforating, the overall wellbore is maintained in an overbalanced condition while a localized underbalanced condition is created at the perforation site that limits the perforating damage, resulting in the productivity benefits of underbalanced perforating, while maintaining the safety and efficiency benefits of an overbalanced wellbore.
In addition to perforating the target hydrocarbon formation, the wellbore itself must typically be conditioned to receive the hydrocarbons from the surrounding formation. In a cased wellbore, this means perforating the wellbore casing as well, which is typically achieved at the same time and in the same manner as the perforation of the surrounding formation. In an open wellbore, the filtercake must be removed from the sides of the open wellbore prior to production. During the drilling of an open wellbore, drilling fluid can be lost by leakage into the formation. To prevent this, a small amount of drilling fluid is often intentionally leaked off to form a hard coating, the filtercake, on the sides of the open wellbore. This filtercake must be removed in order for the well to produce.